Everyone Matters
"Five men and two women, strangers to one another on the eve of that final growing season, now bound by the unspoken promise that the least of them was greater than the sum of all of them." (The Wheat)
"But I don't. I can't. Like I told Zombie on the night Camp Haven blew, the minute we decide one person doesn't matter, they've won, and now my words are the chain that binds me to her." (20)
This theme carries over from the first book, The 5th Wave. In that story, it is evident when Ben decides to go back for Sammy and when Ringer comes back for Ben and Sammy. It illustrates that everyone matters and must be saved. The second that humans stop caring about the weakest of them, humans become like the Others. After that, the battle for humanity's survival will already be over.
Wrong about Aliens
"How baffling it is that we imagined cities incinerated by alien bombs and death rays when all they needed was Mother Nature and time." (9)
This theme also carries over from the first book. Cassie says nearly the same thing in the first chapter of The 5th Wave, and now Ringer echoes it in the first chapter of The Infinite Sea. It highlights the ignorance and tiny imagination of humanity in the face of alien life. Ringer also points out how wrong humanity was to imagine such a bold entrance from the aliens when what they needed was already there.
Don't Be Soft
"Don't go soft." (21)
Ringer constantly tells this to Teacup. She believes that if you become attached to someone or exhibit too much emotion or affection for them, then you are more likely to be killed. Ringer says this despite her secret affection for Teacup, and Teacup's blatant hero worship of her. In the end, it is Teacup's hero worship that causes her to follow Ringer in the woods and accidentally get shot by her, thus endangering both their lives.
Humans are Pests
"And the roar of the Black Hawks' approach makes my thoughts seem like the tiny squeaking whimpers of a dying rodent. It's like the rats, isn't it, Cup? Just like the rats." (22)
Throughout the first book, Cassie constantly compared humans to pests such as cockroaches, with her love interest Evan giving her the nickname mayfly. Now, Ringer constantly compares the human race to rats, scampering around in fear and trying to survive while inevitable death creeps their way. This is used to show the powerlessness as well as the irony of humanity's situation - something once so powerful is now brought so low.
The Imminence of Death
"There's no avoiding death, anyway. Isn't that the lesson they've been hammering into our heads for months? No hiding from it, no running from it. Put it off for a day, and death will surely find your tomorrow." (27)
The squad lives with this fact on a daily basis and it informs every decision that they make. Gone are the days when Ben was just a high school football star and Cassie an awkward girl crushing on him. Now their biggest problems are also the world's biggest problems: how to survive the Others.
Hope is the Key
"He didn't fill me with hate. He filled me with hope." (56)
Ben says this about Vosch, and demonstrates one of the series' central themes: hope is the only thing that humans have left. This was said by Ringer to Ben in the first book, The 5th Wave, and it continues to be a propelling force in the second book, The Infinite Sea.
Run = Die, Stay = Die
"Oh, brother. Guys. I tossed Bear on the desk. 'I've been here before,' I told the Macho Brigade. 'Run equals die. Stay equals die.'" (153)
This is a theme that carries over from the first book as well. There are impossible situations that humans now face due to the circumstances created by the Others. They must work together to find an inventive solution around them, even though all signs point to the fact that they will likely die trying.
